Batteries are the primary energy storage medium on satellites. Long battery life is desired for space applications because the service life of a spacecraft often depends on battery life and excess battery cycle life can be traded for lighter weight. Nickel electrode rechargeable batteries dominate spacecraft energy storage. The dominance of nickel electrode batteries, particularly nickel hydrogen batteries, is expected to continue through at least the year 2010.
Batteries provide power to satellite payload devices during eclipse periods. Depending on the satellite's orbit, some eclipse discharge seasons can last several hundred cycles. However, after repeated charge/discharge cycles, nickel electrode based batteries' discharge voltage performance degrades to lower, unuseful levels. A consequence of battery age is that, in extreme circumstances, satellite eclipse loads cannot be supported at adequate voltages.
Conventionally, lower discharge voltage performance of rechargeable batteries has been corrected by battery reconditioning. Reconditioning is typically accomplished during extended periods of continuous sunlight that do not require load support. Reconditioning is generally not practiced during eclipse because it places the satellite in energy balance jeopardy which can result in spacecraft loss. In reconditioning, battery cells are discharged to near depletion, often over a highly resistive load. For nickel electrode batteries, complete discharge typically converts nickel electrode active material, which has become a bimodal crystal phase structure, back to a single crystal phase material. The single crystal phase active material provides higher discharge voltage performance and an improvement in charge efficiency. However, the positive effects of battery reconditioning are only temporary. Within about 100 charge/discharge cycles, the battery reverts back to its normal bimodal crystal phase structure. The bimodal structure is characterized by having relatively lower charge transfer between the active material's crystal phases and also results in reduced ionic conductance within the battery.
As noted above, depending upon the satellite's orbit, some eclipse discharge seasons can last for several hundred cycles and, for that reason, reconditioning cannot be accomplished with adequate frequency. Moreover, reconditioning is expensive in terms of "ground crew" labor and is also a contributor to increased battery wearout through its high depth of discharge. Furthermore, reconditioning is the primary cause of cell reversal damage and its adverse impact on battery life.
For satellites powered by batteries that suffer from degraded discharge voltage performance and when reconditioning is not feasible, the only option for the satellite is to shed loads by turning off payload devices. The option of shedding loads is disadvantageous because the on-off cycling of payloads accelerates payload device failure. Furthermore, turning off payload devices is economically disadvantageous because the satellite's purpose is payload operation, which is the satellite's base revenue source.
Accordingly, there is a need for improving battery discharge performance. More specifically, there exists a need for improving battery discharge performance without reconditioning. This need exists for improving battery discharge performance aboard any spacecraft that experiences long eclipse seasons and high battery discharge demands. The present invention seeks to fulfill these needs and provides further related advantages.